1. Field of the Invention
The present invention relates to an improved isomerization process. More particularly, the invention concerns the isomerization of butane to isobutane with strong acid catalyst systems at low temperatures and short contact times.
2. Description of the Prior Art
The isomerization of n-butane to isobutane is of substantial practical significance. n-Butane is available from natural gas sources which contain only very limited amounts of the branched chain isomer which is essential in the manufacture of polyisobutylene, tert-butyl alcohol and other products.
The isomerization of n-butane to isobutane is generally carried out by using an acidic catalyst such as aluminum chloride or a supported noble metal catalyst capable of isomerization via dehydrogenation-hydrogenation. Industrial processes operate in the 100.degree.-200.degree. C. temperature range. The two major available processes are the UOP and BP processes. The UOP process operates with a platinum containing catalyst at 250.degree.-380.degree. C. whereas the BP isomerization process uses a platinum catalyst and operates between 15 and 30 atms pressure at a temperature of between about 150.degree.-200.degree. C. Under these conditions, n-butane feed under hydrogen pressure is brought to near equilibrium conversion to isobutane. The liquid product which contains normal butane and isobutane is separated from the cooled reactor effluent gas which is recycled to the reactor with hydrogen makeup. After stabilization, the liquid is passed to the deisobutanizer column from which an isobutane overhead product is taken. The lower product, normal butane, is recycled to the reactor loop. The bottom product from the deisobutanizer is a C.sub.5 + reject which can be used as a low octane by-product.
The discoveries of superacidic systems (i.e., acids many million times stronger than 100% H.sub.2 SO.sub.4 or anhydrous hydrogen fluoride) by G. A. Olah opened up extensive research into superacid catalyzed hydrocarbon conversions, including the isomerization of alkanes. U.S. Pat. No. 3,708,553 and U.S. Pat. No. 3,766,286 disclose processes for the alkylation of hydrocarbons with alkenes and the isomerization of hydrocarbons, respectively, utilizing superacidic catalysts of one or more Lewis acid halides, preferentially fluorides such as antimony pentafluoride, tantalum pentafluoride, niobium pentafluoride, vanadium pentafluoride, titanium tetrafluoride, molybdenum hexafluoride, bismuth pentafluoride, phosphorus pentafluoride, arsenic pentafluoride, and the like, selected from Group IV-B, V and VI-B elements of the Periodic Table and a strong Bronsted acid such as fluorosulfonic acid, trifluoromethanesulfonic acid, and trifluoroacetic acid. Numerous additional patents describe variations of the superacid catalyzed hydrocarbon transformations and include, for example, the following:
U.S. Pat. No. 3,839,489 discloses isomerization of normal paraffinic hydrocarbons having from 4 to 8 carbon atoms with a catalyst system comprising a Group V-B fluoride, such as arsenic or antimony pentafluoride, and trifluoromethanesulfonic acid or hydrogen fluoride under hydrogen partial pressures ranging from 900 to 4000 psig.
U.S. Pat. No. 3,852,184 discloses the isomerization of alkylcyclopentanes in a reforming feed with a catalyst comprising a metal halide and protonic acid wherein the protonic acid/metal halide molar ratio is at least 2:1 and preferably 5:1.
U.S. Pat. No. 3,855,346 discloses a process for the isomerization of C.sub.4 -C.sub.7 normal and branched chain paraffins with a catalyst comprising trifluoromethanesulfonic acid alone or in combination with a metal fluoride of a Group V-B element such as antimony pentafluoride, arsenic pentafluoride and phosphorus pentafluoride.
U.S. Pat. No. 4,144,282 upgrades light naphtha streams using a fluoroalkanesulfonic acid-antimony pentafluoride mixture as a conversion catalyst.
A description of superacids and hydrocarbon transformation reactions, including isomerization, is disclosed in "Superacids" by George A. Olah, G. K. Surya Prakash and Jean Sommer, Science, Oct. 5, 1979, Vol. 206, No. 4414.
In the acid catalyzed isomerization of alkanes, n-butane isomerization represents a special and more difficult problem than that of pentanes, hexanes, heptanes, etc. The reason for this is that in the isomerization of n-butane to isobutane the reaction is either bimolecular, in the presence of butanes, or intramolecular, where it must proceed through a primary isobutyl cation, a relatively high energy species, which consequently raises the activation energy of the process. There is, however, no .beta.-cleavage involved in the C.sub.4 + system. In contrast, however, acid catalyzed protolytic cleavage of the .alpha. and .beta. C--C bonds in n-butane is a serious competing reaction, particularly at high superacidities. This is the reason, for example, why Brouwer et al reported (Rec. Trav. Chim. 87, 1435 (1968)), that the extremely strong 1:1 HF-SbF.sub.5 system is not isomerizing n-butane to isobutane, but causes only carbon/and hydrogen scrambling and hydrocracking, whereas higher alkanes are readily isomerized. Similarly, the Magic Acid (1:1 FSO.sub.3 H-SbF.sub.5) catalyzed treatment of n-butane results in extensive hydrocracking and only limited isomerization. Oxidative isomerization with SO.sub.3 in fluoro- and chlorosulfuric acid, similar to Magic Acid, is disclosed by M. P. Herlem, in French Pat. No. 2,252,351, (CA 84 43277x) but only for alkanes of at least 5 carbon atoms.